1. Field of the Invention
The present invention relates to protection of a switching element for controlling the power supply to a motor drive winding from a short circuit.
2. Description of Related Art
In a switching IC for driving a winding of a motor by application of a relatively large current, the switching element sometimes is overloaded and broken down due to a short circuit between an output terminal and a GND terminal of a switching element (hereinafter, referred to as a “ground fault”) or a short circuit between the output terminal and a power terminal of the switching element (hereinafter, referred to as a “power supply fault”) that is caused by, for example, a solder bridge created when the IC is mounted on a substrate.
As a solution to this problem, an exemplary power supply circuit provided with a short circuit protection control device is disclosed in JP 2005-252763 A. FIG. 6 is a block circuit diagram showing an exemplary configuration of the conventional power supply circuit. This circuit is divided into a short circuit protection control device 1 and a switching circuit 2. An upper switching element 3 and a lower switching element 4 with a MOS structure are connected in series between a power source VM and GND. An upper pre-drive circuit 6 is connected to a gate of the upper switching element 3, and a lower pre-drive circuit 8 is connected to a gate of the lower switching element 4. The upper pre-drive circuit 6 turns the upper switching element 3 ON/OFF based on an upper control signal 5. The lower pre-drive circuit 8 turns the lower switching element 4 ON/OFF based on a lower control signal 7.
Further, an upper breaking circuit 9 and a lower breaking circuit 10 for respectively turning the upper switching element 3 and the lower switching element 4 OFF are connected to the gates of the respective switching elements 3 and 4. The upper breaking circuit 9 and the lower breaking circuit 10 are controlled by output signals of a ground fault detection circuit 11 and a power supply fault detection circuit 12, respectively.
The ground fault detection circuit 11 includes a two-input ground fault detection comparator 13 whose positive-side input is supplied with an arbitrary voltage V1 and whose negative-side input is connected with a node 14 between the upper switching element 3 and the lower switching element 4. Further, the ground fault detection comparator 13 is supplied with a power source VC via a switch SW1. In a state where the switch SW1 is turned ON, so that the power source VC is supplied to the ground fault detection comparator 13, when the voltage of the node 14 is lower than the voltage V1, the ground fault detection comparator 13 causes the upper breaking circuit 9 to turn OFF the upper switching element 3.
The power supply fault detection circuit 12 similarly includes a two-input power supply fault detection comparator 16 whose positive-side input is supplied with an arbitrary voltage V2 and whose negative-side input is connected with the node 14. Further, the power supply fault detection comparator 16 is supplied with a power source VC via a switch SW2. In a state where the switch SW2 is turned ON, so that the power source VC is supplied to the power supply fault detection comparator 16, when the voltage of the node 14 is higher than the voltage V2, the power supply fault detection comparator 16 causes the lower breaking circuit 10 to turn OFF the lower switching element 4.
Further, an upper current monitor circuit 15 and a lower current monitor circuit 17 are connected to the gates of the upper switching element 3 and the lower switching element 4, respectively. The switch SW1 is turned ON/OFF based on an output of the upper current monitor circuit 15, thereby controlling the supply of the power source VC for operating the ground fault detection comparator 13. More specifically, when a current value of the upper switching element 3 is not less than a level at which the switching element 3 is assumed to be broken down, the switch SW1 is turned ON, so that the ground fault detection comparator 13 is supplied with the power source VC to be operated. On the other hand, when the current value is below such a level, the switch SW1 is turned OFF, so that the ground fault detection comparator 13 is not supplied with the power source VC and is in a non-operating state. Accordingly, the upper breaking circuit 9 is not operated, resulting in a normal operation of the upper switching element 3.
Meanwhile, the power source VC for operating the power supply fault detection comparator 16 is controlled by the switch SW2 based on an output of the lower current monitor circuit 17. More specifically, when a current value of the lower switching element 4 is not less than a level at which the switching element 4 is assumed to be broken down, the switch SW2 is turned ON, so that the power supply fault detection circuit 12 is supplied with the power source VC to be operated. On the other hand, when the current value is below such a level, the switch SW2 is turned OFF, so that the power supply fault detection circuit 12 is not supplied with the power source VC and is in a non-operating state. Accordingly, the lower breaking circuit 10 is not operated, resulting in a normal operation of the lower switching element 4.
With the above-described circuit configuration in mind, consideration is given to the case where the node 14 is short-circuited to ground when the upper switching element 3 is ON. FIG. 7 shows the case where the node 14 is short-circuited to ground.
At this time, since the output terminal (node 14) is short-circuited to ground and has a GND potential, an excess current of an amount of (power source VM)/(ON-resistance of upper switching element 3) flows through the upper switching element 3 toward the node 14 as a point of ground fault. In other words, the upper switching element 3 carries an unexpected overload. Thus, the upper current monitor circuit 15 detects the excess current, and turns the switch SW1 ON, so that the power source VC is supplied to the ground fault detection comparator 13.
Meanwhile, the power supply fault detection comparator 16 is in a non-operating state since no current flows through the switching element 4 and the switch SW2 is turned OFF by the lower current monitor circuit 17.
Since the node 14 is short-circuited to ground, a voltage of substantially 0 is input to the negative-side input of the ground fault detection comparator 13. In the case where the voltage V1 to be supplied to the positive-side input is the power source VM/2, for example, the ground fault detection comparator 13 is supplied with a higher voltage to the positive-side input than to the negative-side input. Accordingly, the upper breaking circuit 9 is operated and turns the upper switching element 3 OFF. Consequently, the excess current flowing through the upper switching element 3 is interrupted, thereby preventing a breakdown of the upper switching element 3 due to the excess current.
On the other hand, consideration is given to the case where the node 14 is short-circuited to the power supply (power source VM) when the lower switching element 4 is ON. FIG. 8 shows the case where the node 14 is short-circuited to the power supply.
At this time, since the output terminal (node 14) is short-circuited to the power supply and has a potential VM, an excess current of an amount of (power source VM)/(ON-resistance of lower switching element 4) flows through the lower switching element 4 from the node 14 as a point of power supply fault. In other words, the lower switching element 4 carries an unexpected overload. Thus, the lower current monitor circuit 17 detects the excess current, and turns the switch SW2 ON, so that the power source VC is supplied to the power supply fault detection comparator 16.
Meanwhile, the ground fault detection comparator 13 is in a non-operating state since no current flows through the switching element 3 and the switch SW1 is turned OFF by the upper current monitor circuit 15.
Since the node 14 is short-circuited to the power supply, substantially the same voltage as that of the power source VM is supplied to the positive-side input of the power supply fault detection comparator 16. In the case where the voltage V2 to be supplied to the negative-side input is the power source VM/2, for example, the power supply fault detection comparator 16 is supplied with a higher voltage to the positive-side input than to the negative-side input. Accordingly, the lower breaking circuit 10 is operated and turns the lower switching element 4 OFF. Consequently, the excess current flowing through the lower switching element 4 is interrupted, thereby preventing a breakdown of the lower switching element 4 due to the excess current.
Next, a description will be given of the case where motor drive windings of three phases are to be driven by the switching elements, with reference to the drawings. FIG. 9 is a block circuit diagram showing an exemplary configuration of the conventional short circuit protection control device in the case where motor drive windings of three phases, i.e., the U-phase, the V-phase, and the W-phase, are to be driven.
In FIG. 9, a three-phase bridge circuit including upper switching elements 1U, 1V, and 1W and lower switching elements 2U, 2V, and 2W for the U-phase, the V-phase, and the W-phase is configured between a power source VM and GND. Their output terminals 3U, 3V, and 3W are connected with motor drive windings 4U, 4V, and 4W to be driven. RU, RV, and RW represent resistance components of the respective windings.
Circuits for driving the upper switching elements 1U, 1V, and 1W and the lower switching elements 2U, 2V, and 2W for the U-phase, the V-phase, and the W-phase each have the same configuration as that of the above-described short circuit protection control device 1, and short circuit protection control devices 5U, 5V, and 5W are provided respectively for the U-phase, the V-phase, and the W-phase. The following description is directed only to a specific configuration of the short circuit protection control device 5U.
The short circuit protection control devices 5U, 5V, and 5W are turned ON/OFF based on U-phase, V-phase, and W-phase upper control signals 6U, 6V, and 6W and U-phase, V-phase, and W-phase lower control signals 7U, 7V, and 7W, respectively. Since the configuration and operation of each of the short circuit protection control devices 5U, 5V, and 5W are the same as those of the above-described short circuit protection control device 1, descriptions thereof will be omitted.
With the above-described circuit configuration in mind, consideration is given to the case where energization of the motor drive windings 4U, 4V, and 4W is started with the U-phase upper switching element 1U and the W-phase lower switching element 2W turned ON from a non-energized state and then the output terminal 3W is short-circuited to ground (GND). FIG. 10 shows the case where the node 3W is short-circuited to ground.
As shown by a broken line in FIG. 10, a current flows from the U-phase upper switching element 1U through the motor drive windings 4U and 4W to the point of W-phase ground fault.
At this time, since the output terminal 3W is short-circuited to ground and has a GND potential, an excess current of VM/(Ron+RU+RW), where Ron is an ON resistance of the U-phase upper switching element 1U, flows through the U-phase upper switching element 1U. As a result, the U-phase upper switching element 1U carries an unexpected overloaded. Thus, the upper current monitor circuit 15 detects the excess current, and turns the switch SW1 ON, so that the power source VC is supplied to the ground fault detection comparator 13.
The voltage of the output terminal 3U to be input to the ground fault detection comparator 13 is expressed by (RU+RW)/(Ron+RU+RW)×VM. For example, when the winding resistance RU or RW of the motor drive winding is sufficiently lower than the resistance Ron, the voltage of the output terminal 3U is substantially equal to the GND voltage, and the upper breaking circuit 9 is operated by the ground fault detection comparator 13 so as to turn the U-phase upper switching element 1U OFF. Consequently, the excess current flowing through the U-phase upper switching element 1U is interrupted, thereby preventing a breakdown of the U-phase upper switching element 1U.
However, in the case where the winding resistance RU or RW is sufficiently higher than the ON-resistance Ron of the U-phase upper switching element 1U, the voltage of the output terminal 3U is substantially equal to the power source VM. Since the ground fault detection comparator 13 is supplied with VM/2 to the positive-side input and the power source VM to the negative-side input, the upper breaking circuit 9 is not operated despite the fact that the output terminal 3W is short-circuited to ground, and thus the U-phase upper switching element 1U remains in an ON state. As a result, the U-phase upper switching element 1U could be overloaded continuously and broken down, and energization of the motor drive winding could be started in an abnormal state where the output terminal 3U is short-circuited to ground.
As described above, in the exemplary conventional configuration, detection of a ground fault depends on the resistance component of the drive target, which sometimes makes it impossible to detect a ground fault.